(1) Field of the Invention
The present invention relates to an epoxy resin composition. More particularly, the present invention relates to an epoxy resin composition excellent in the heat resistance and flame retardancy, which is obtained by reacting a specific trifunctional epoxy compound with a halogenated bisphenol. Furthermore, the present invention relates to a novel epoxy resin excellent in the heat resistance and mechanical properties.
(2) Description of the Prior Art
Compositions formed by incorporating a curing agent such as an aromatic polyamine, an aliphatic polyamine, a polyamide-amine, an amine adduct, dicyandiamide, an acid anhydride or a phenol-novolak resin into an epoxy resin have been used as adhesives, paints, molding materials and casting materials. Moreover, it is known that a varnish is formed by dissolving such a composition in a solvent, a reinforcing substrate is impregnated or coated with this varnish and the impregnated or laminated reinforcing substrate is used for molding a laminated plate.
Recently, in the electrical and electronic fields, with required reduction of the size and required increase of the precision, improvement of the heat resistance is eagerly desired in adhesives, insulating paints, sealants and laminated plates for electronic parts for enhancing the reliability at high-temperature applications. More specifically, an adhesive, paint, sealant or laminated plate prepared by using a commercially available bisphenol A type epoxy resin is generally low in the heat distortion temperature or electrical insulating property and, therefore, the reliability is poor.
Furthermore, a high flame retardancy is required for materials used in the electrical or electronic field. As the flame-retardant epoxy resin used as a laminated plate (printed circuit substrate comprising a laminate of a glass cloth and an epoxy resin), there is known, for example, a linear epoxy resin obtained by reacting a bisphenol A type epoxy resin such as a liquid bisphenol A type epoxy resin having an epoxy equivalent of about 190 with tetrabromobisphenol A. When this linear epoxy resin is cured with, for example, dicyandiamide which is a curing agent used for formation of a laminated plate and having a high general-purpose property, the glass transition temperature (Tg) of the cured product (the bromine content is 20 to 22% by weight) is as low as 120.degree. to 130.degree. C. If a large amount of a polyfunctional epoxy resin such as o-cresol-novolak epoxy resin or a phenol-novolak epoxy resin is added to the above epoxy resin so as to increase the heat resistance of the cured product, the flame retardancy is reduced and the moldability becomes insufficient and, therefore, the amount added of the polyfunctional epoxy resin is restricted.
As is apparent from the foregoing description, in the known epoxy resins, the heat resistance and flame retardancy are properties contradictory to each other, and an epoxy resin excellent in both the heat resistance and flame retardancy is not known. In the electronic field where high performance is required, in order to improve the reliability of a cured product at high temperatures, development of an epoxy resin excellent in both the heat resistance and flame retardancy is eagerly desired.
A cured product formed from a polyfunctional epoxy resin such as an o-cresol-novolak epoxy resin or a phenol-novolak resin has a high flexural modulus and hence, is hard and brittle and, therefore, the cured product is poor in the mechanical properties and cracking is readily caused by a thermal shock. Accordingly, development of an epoxy resin excellent in both the heat resistance and mechanical properties is eagerly desired.